I was surprised that no one in this thread asked "What are you using it for?"
If you are going to shoot CQB all Spring/Summer/Fall (and early winter) long and go through 1000's of rounds a year, then you should be a bit more concerned about the quality of the parts and warranties and such.
If you are going to plink at the range and fire <1000 rounds/year, any gun mentioned in this thread will be fine.
If you want to get into Service Rifle, and plan on shooting distance, then you might consider either a better model, or at least an upgraded upper.
The other thing that hasn't come up is that you can start with any AR and modify it as you go along. Lots of guys run the fancy new upper they just bought on their old Nork lower (sometimes a *very slight* bit of work is needed to make them mate, but usually not, and this is another thread entirely).
The main thing is to get something in your budget, and shoot it. There is no platform more customizable than the AR.
Finally, the guys that mentioned the EE are spot-on. There are lots of GREAT rifles on there currently, in your price range. Check it out.
B.